1. Field of the Invention
The present invention relates generally to a display element, and more particularly to a display element which performs optical modulation by utilizing the phenomenon of optical diffraction.
In addition, the present invention relates generally to an observation apparatus including a display apparatus having the aforesaid display element, and more particularly to an observation apparatus adapted for use in the viewfinder of a camera which enables an operator to observe the image of an object and a display pattern both of which are optically superimposed on each other.
2. Related Background Art
A typical light modulating element which has heretofore been known employs a so-called TN (twist-nematic) type liquid crystal which essentially comprises: a pair of polarizing plates arranged such that their respective directions of vibration of polarization are cross perpendicular to each other; an element disposed in the gap between the pair of polarizing plates and having a pair of optical transparent substrates in which a liquid crystal is enclosed, the facing surface of the pair of substrates being respectively subjected to orientation patterning. The liquid crystal is switched over between the state of being twisted and the state of being oriented perpendicular to the substrate surface, thereby optically modulating incident light.
Since such a display element has a simple construction and is easy to drive, this type is widely used in various fields. However, the aforesaid two polarizing plates must be used so as to transmit and cut off luminous flux, so that the transmittance of the element is inferior. It is therefore known that this type is not a suitable light modulating element from the viewpoints of the efficiency of utilization of luminous flux.
A so-called guest-host-mode liquid crystal is also known as a display element of the same type that utilizes a liquid crystal and in which liquid-crystal molecules and coloring matter are mixed together. Since this type display element includes additional coloring material, the transmittance thereof is at best about 70% even at the time of color extinction.
In addition, Japanese Patent Examined Publication No. 3932/1978 and U.S. Pat. No. 4,251,137 disclose a variable diffractive substractive color filter element and a display element formed by a combination of a liquid crystal and a reflection type or a translucent phase type diffraction grating. Since these elements disclosed in the prior art are respectively capable of utilizing luminous flux with high efficiency, they are usable for a display element incorporated in the viewfinder of a camera, a light bulb or the like.
However, the aforementioned display element utilizing the diffraction grating has the problem that multiple reflection or unwanted diffracted light is produced within a display pattern, and the thus-produced unwanted light contributes to lowering display quality.
The display element including such a light modulating element is typically disposed in the vicinity of, for example, the focal plane of the viewfinder system of a camera, and the operator might observe a high intensity object (for example, a mercury lamp) through such a viewfinder system. In this case, if the light from the high intensity object is incident upon any part of the diffraction grating portion which is displaying an image, a rainbow-like diffracted image may be formed in the remaining region of the same diffraction grating portion. This leads to a problem in that the rainbow-like image lowers the quality of the display element.